Liquid crystal display device

ABSTRACT

The present invention provides a liquid crystal display device that, with photo-alignment films, can maintain a favorable voltage holding ratio for a long period of time and prevent generation of image sticking and stains on the display screen. The liquid crystal display device includes: an active-matrix liquid crystal panel; and a backlight, the liquid crystal panel including a liquid crystal layer, paired substrates holding the liquid crystal layer in between, and alignment films disposed on the liquid crystal layer side surfaces of the respective substrates, the alignment films each being a photo-alignment film formed from a material exhibiting photo-alignment performance, the liquid crystal layer containing a liquid crystal material and a radical scavenger.

TECHNICAL FIELD

The present invention relates to liquid crystal display devices. Morespecifically, the present invention relates to a liquid crystal displaydevice configured to control the alignment of liquid crystal moleculeswith alignment films.

BACKGROUND ART

Liquid crystal display devices are display devices utilizing a liquidcrystal composition for display. The typical display mode thereof isirradiating a liquid crystal panel containing a liquid crystalcomposition sealed between paired substrates with backlight illuminationand applying voltage to the liquid crystal composition to change thealignment of the liquid crystal molecules, thereby controlling theamount of light passing through the liquid crystal panel. Such liquidcrystal display devices have features including a thin profile, lightweight, and low power consumption, and have therefore been used forelectronic devices such as smartphones, tablet PCs, and car navigationsystems. The pixel resolution has been increased for uses such assmartphones, which has led to a tendency of an increase in the number ofconductive lines and the area of the black matrix disposed in the liquidcrystal panel.

In a liquid crystal display device, the alignment of liquid crystalmolecules with no voltage applied is typically controlled by alignmentfilms on which an alignment treatment has been performed. The alignmenttreatment has conventionally been performed by the rubbing method ofrubbing the surface of an alignment film with a tool such as a roller.However, since the number of the conductive lines and the area of theblack matrix disposed in the liquid crystal panel have been increased,irregularities are now more likely to occur on the substrate surfaces inthe liquid crystal panel. With irregularities on the substrate surfaces,the portions near the irregularities may not be properly rubbed by therubbing method. Such a non-uniform alignment treatment may cause adecrease in the contrast ratio in the liquid crystal display device.

In order to deal with this problem, studies and development have beenmade on a photo-alignment method which is an alternative alignmenttreatment method to the rubbing method and irradiates the surface of analignment film with light. With the photo-alignment method, an alignmenttreatment can be performed without contact with the surface of thealignment film. The photo-alignment method therefore has an advantagethat alignment treatment is less likely to be uneven even withirregularities on a substrate surface, so that a favorable liquidcrystal alignment can be achieved on the entire substrate.

Moreover, the increase in the number of conductive lines and the area ofthe black matrix disposed in the liquid crystal panel may decrease thearea ratio of openings usable for display (aperture ratio). Such adecrease in the aperture ratio will be a direct cause of a decrease inthe amount of light that can pass through the liquid crystal panel.Significantly increasing the luminance of the backlight has thereforebeen considered to maintain the display performance, including thecontrast ratio, of liquid crystal display devices.

Meanwhile, liquid crystal compositions used for liquid crystal displaydevices have been desired to have higher stability such that thecompositions can withstand the load in the production processes ofliquid crystal display devices and contribute to long-term stability ofthe produced liquid crystal display devices. For example, PatentLiterature 1 discloses addition of an antioxidant and a light stabilizerto the liquid crystal composition. Patent Literature 2 also disclosesaddition of a stabilizer to the liquid crystal composition (see Table Cin paragraphs [0208] to [0211]).

CITATION LIST Patent Literature

Patent Literature 1: JP 2007-197731 A

Patent Literature 2: JP 2011-515543 T

SUMMARY OF INVENTION Technical Problem

To respond to the increased pixel resolution, the photo-alignment methodhas been developed and the luminance of the backlight has beenincreased. These attempts, however, have been found to result in anincreased tendency of occurrence of stains (unevenness) at the edges ofthe screen of a liquid crystal panel and at the edges of a displayed boxpattern. Such defects at the edges of a displayed box pattern aredetected as image sticking.

The inventors of the present invention have made various studies, andcame to an idea that the image sticking and stains described above occurthrough the following steps.

(1) Generation of Radicals

Irradiation of the liquid crystal panel with backlight illumination(amount of energy: hν) excites a photo-functional group contained in thephoto-alignment films as shown in the following scheme (A-I), causingthe photo-functional group to cleave and thereby generate radicals.Especially in the case of using a backlight with an increased luminance,the generation of radicals is noticeable.

(2-1) First Ion Generation

The radicals generated in the photo-alignment films are released intothe liquid crystal layer, and the released radicals are ionized.

(2-2) Second Ion Generation

The radicals generated in the photo-alignment films are released intothe liquid crystal layer and transfer from the photo-functional groupsto the liquid crystal molecules, so that the liquid crystal molecule areionized.

(3) Decrease in Voltage Holding Ratio

The ions in the liquid crystal layer accumulate at the edges of thescreen of the liquid crystal panel or at the edges of a displayed boxpattern. The voltage holding ratio (VHR) at these edges decreases tocause the image sticking and stains described above.

As described above, some conventional liquid crystal compositionscontain additives such as an antioxidant and a light stabilizer. Theseadditives, however, could not solve the problems unique to the use ofphoto-alignment films. That is, in liquid crystal display devices, inthe case that oxygen in the outside enters the liquid crystal panel tooxidize the liquid crystal material, the oxidants may unfortunatelycause image sticking and stains in the displayed images. In order toprevent these defects, additives such as an antioxidant having afunction of eliminating oxygen from the oxidants generated under lightor heat in the presence of oxygen have conventionally been added to theliquid crystal composition. Yet, in the case that radicals are generatedfrom the photo-alignment films and react with the antioxidant, thereaction consumes the antioxidant. The antioxidant therefore fails tofunction as intended, allowing oxidation of the liquid crystal moleculesand the alignment films to proceed. The oxidants generated here may alsobe ionized to cause a decrease in the voltage holding ratio.

The present invention has been made in view of such a current state ofthe art, and aims to provide a liquid crystal display device that, withphoto-alignment films, can maintain a favorable voltage holding ratiofor a long period of time and prevent generation of image sticking andstains on the display screen.

Solution to Problem

The inventors of the present invention have focused on the decrease inthe voltage holding ratio at the edges of the screen of the liquidcrystal panel and the edges of a displayed box pattern in a liquidcrystal display device with photo-alignment films, which causes defectssuch as image sticking and stains on the display screen. The inventorsof the present invention have then made intensive studies, and havefirst found that the defects are caused by a phenomenon in whichradicals are generated in the photo-alignment films exposed to thebacklight illumination and the generated radicals are released into theliquid crystal layer. The inventors have found that this problem can besolved by adding a radical scavenger to the liquid crystal layer,thereby completing the present invention.

One aspect of the present invention may be a liquid crystal displaydevice, including: an active-matrix liquid crystal panel; and abacklight. The liquid crystal panel may include a liquid crystal layer,paired substrates holding the liquid crystal layer in between, andalignment films disposed on the liquid crystal layer side surfaces ofthe respective substrates. The alignment films may each be aphoto-alignment film formed from a material exhibiting photo-alignmentperformance. The liquid crystal layer may contain a liquid crystalmaterial and a radical scavenger.

Advantageous Effects of Invention

The liquid crystal display device of the present invention, having theabove configuration, can deactivate the radicals released into theliquid crystal layer with the radical scavenger, preventing a decreasein the voltage holding ratio. Thus, the liquid crystal display device,with photo-alignment films, can maintain a favorable voltage holdingratio for a long period of time and prevent generation of image stickingand stains on the display screen.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view schematically illustrating a liquidcrystal display device of the present embodiment.

FIG. 2 is a view describing the reaction mechanism of deactivatingradicals generated from photo-alignment films with a hindered aminecompound (radical scavenger).

FIG. 3 is a view describing the effect of a phenolic antioxidant in thepresent invention.

FIG. 4 is a view describing the reaction between a phenolic antioxidantand a photo-reactive film.

FIG. 5 is a graph showing changes over time in the voltage holding ratioof the liquid crystal panels of Example 1 and Comparative Examples 1 and2.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the present invention is described. Thecontents of the following embodiment are not intended to limit the scopeof the present invention, and the design may appropriately be changedwithin the spirit of the configuration of the present invention.

FIG. 1 is a cross-sectional view schematically illustrating a liquidcrystal display device of the present embodiment. The liquid crystaldisplay device of the present embodiment includes an active-matrixliquid crystal panel 20 and a backlight 10. The liquid crystal panel 20includes a liquid crystal layer 23, paired substrates 21 holding theliquid crystal layer 23 in between, and alignment films 22 disposed onthe liquid crystal layer 23 side surfaces of the respective substrates21. The alignment films 22 each are a photo-alignment film formed from amaterial exhibiting photo-alignment performance. The liquid crystallayer 23 contains a liquid crystal material and a radical scavenger.

The active-matrix liquid crystal panel 20 may be any active-matrixliquid crystal panel including the liquid crystal layer 23, thesubstrates 21 holding the liquid crystal layer 23 in between, and thealignment films 22 disposed on the liquid crystal layer 23 side surfacesof the respective substrates 21, and can be a common liquid crystalpanel in an active-matrix display mode. In the active-matrix displaymode, usually, signal voltage is applied to the electrodes throughthin-film transistors (TFTs) while the active elements such as the TFTsin the respective pixels are on, and the charges supplied to the pixelsare held while the active elements are off. The ratio of holding thesupplied charges during one frame (for example, 16.7 ms) is the voltageholding ratio (VHR). That is, a low VHR means that the voltage appliedto the liquid crystal layer is likely to decay over time. Thus, in theactive-matrix display mode, a high VHR is desired.

Examples of the paired substrates 21 include a combination of an activematrix substrate (TFT substrate) and a color filter (CF) substrate. Theactive matrix substrate can be one commonly used in the field of liquidcrystal display devices. The configuration of the active matrixsubstrate in a plan view may be one including, on a transparentsubstrate, gate signal lines parallel to each other; source signal linesthat extend in the direction orthogonal to the gate signal lines and areparallel to each other; active elements such as TFTs disposed on therespective intersections of the gate signal lines and the source signallines; and pixel electrodes disposed in a matrix form in the respectiveregions defined by the gate signal lines and the source signal lines,for example. In the case of a horizontal alignment mode, theconfiguration further includes common lines and counter electrodesconnected to the common lines, for example. Suitable examples of theTFTs include those including a channel made of an oxide semiconductor,indium-gallium-zinc-oxide (IGZO).

The color filter substrate can be one commonly used in the field ofliquid crystal display devices. The configuration of the color filtersubstrate may be one including, on a transparent substrate, a blackmatrix formed in a grid pattern, and color filters formed inside therespective grids, i.e., pixels, for example.

Here, both the color filters and the active matrix may be formed on oneof the paired substrates 21.

The substrates 21 and the liquid crystal layer 23 hold the respectivealignment films 22 in between. Each alignment film 22 has a function ofcontrolling the alignment of liquid crystal molecules in the liquidcrystal layer 23. With voltage lower than the threshold voltage appliedto the liquid crystal layer 23 (including the case of no voltageapplication), the liquid crystal display device mainly utilizes thealignment films 22 to control the alignment of the liquid crystalmolecules in the liquid crystal layer 23. The angle of the major axis ofeach liquid crystal molecule from the surface of one of the substrates21 in such a controlled state is referred to as a “pre-tilt angle”. The“pre-tilt angle” as used herein refers to the angle of tilt of liquidcrystal molecules from the direction parallel to the substrate surface,with the angle direction parallel to the substrate surface being 0° andthe angle direction which is the same as the substrate surface normaldirection being 90°.

The alignment films 22 may provide any pre-tilt angle to the liquidcrystal molecules. The alignment films 22 may be horizontal alignmentfilms or vertical alignment films, but are preferably horizontalalignment films. In the case that the alignment films 22 are horizontalalignment films, the pre-tilt angle is preferably substantially 0°(e.g., smaller than 10°), more preferably 0° for achievement of theeffect of maintaining favorable contrast characteristics for a longperiod of time. In the case that the display mode is the IPS mode or FFSmode, the pre-tilt angle is also preferably 0° from the viewpoint ofviewing angle characteristics, whereas in the case that the display modeis the TN mode, the pre-tilt angle is preferably set to about 2°, forexample, due to the restrictions in the mode.

The alignment films 22 are photo-alignment films formed from a materialexhibiting photo-alignment performance. The material exhibitingphoto-alignment performance means any general material that, whenirradiated with light (electromagnetic waves) such as ultraviolet lightor visible light, undergoes a structural change to exhibit performance(alignment force) of controlling the alignment of the nearby liquidcrystal molecules, or to change in the alignment force power and/ordirection.

Examples of the material exhibiting photo-alignment performance includethose including a photo-reactive site that undergoes a reaction such asdimerization (dimer formation), isomerization, photo-Friesrearrangement, or decomposition, when irradiated with light. Suitableexamples of the photo-reactive site (functional group) that undergoesdimerization and isomerization when irradiated with light includecinnamate represented by the following formula (B-1), 4-chalconerepresented by the following formula (B-2-1), 4′-chalcone represented bythe following formula (B-2-2), coumarin represented by the followingformula (B-3), and stilbene represented by the following formula (B-4).The following scheme (B-1-I) shows the isomerization and dimerizationreactions of cinnamate.

Suitable examples of the photo-reactive site (functional group) thatundergoes isomerization when irradiated with light include azobenzene.The following formula (B-5-1) shows trans-azobenzene, and the followingformula (B-5-2) shows cis-azobenzene.

Suitable examples of the photo-reactive site that undergoes photo-Friesrearrangement when irradiated with light include a phenolic esterstructure represented by the following formula (B-6). The phenol esterstructure undergoes photo-Fries rearrangement as shown in the followingscheme (B-6-I).

Suitable examples of the photo-reactive site that undergoesdecomposition when irradiated with light include a cyclobutanestructure. Examples of photo-alignment films containing a cyclobutanestructure include a polymer obtained by copolymerizing the followingmonomers: an acid anhydride that is represented by the following formula(B-7-1) and contains a cyclobutane structure; and an amine compoundrepresented by the following formula (B-7-2). As shown in the followingscheme (B-7-I), the cyclobutane structure in this polymer cleaves whenirradiated with light, exhibiting photo-alignment performance. Thehydrogen atoms in the cyclobutane structure shown in the followingformula (B-7-1) may each be replaced by another atom or a functionalgroup.

The present embodiment may employ a polymer sustained alignment (PSA)technique. The PSA technique fixes the initial tilt (pre-tilt) of theliquid crystal using a polymer. The polymer here is formed on thesurfaces of the alignment films 22 by sealing a liquid crystalcomposition containing a photo-polymerizable monomer between the pairedsubstrates 21, and irradiating the resulting liquid crystal layer 23with light to polymerize the photo-polymerizable monomer.

Examples of a configuration employing the PSA technique include oneincluding, on the liquid crystal layer 23 side surface of each of thealignment films 22, a layer containing a polymer tat is obtained bypolymerizing a photo-polymerizable monomer represented by the followingformula (C):A1-Y-A2  (C)wherein Y represents a structure containing at least one benzene ringand/or a condensed benzene ring; hydrogen atoms in the benzene ring andthe condensed benzene ring may each be replaced by a halogen atom; atleast one of A1 and A2 represents an acrylate or a methacrylate; and A1and A2 each directly bind to the benzene ring or the condensed benzenering.

The skeleton Y in the above formula (C) is preferably a structurerepresented by the following formula (C-1), (C-2), or (C-3). Thehydrogen atoms in the following formulas (C-1), (C-2), and (C-3) mayeach individually be replaced by a halogen atom.

Specific examples of the photo-polymerizable monomer represented by theabove formula (C) include those represented by the following formula(C-1-1), (C-1-2), or (C-3-1).

In the present embodiment, the liquid crystal layer 23 contains a liquidcrystal material and a radical scavenger.

<Liquid Crystal Material>

The liquid crystal material may have a negative or positive value forthe anisotropy of dielectric constant (Δε) defined by the followingformula (P). That is, the liquid crystal material may have negativeanisotropy of dielectric constant or positive anisotropy of dielectricconstant. The liquid crystal material having negative anisotropy ofdielectric constant may be one with an anisotropy of dielectric constantΔε of −1 to −20, for example. The liquid crystal material havingpositive anisotropy of dielectric constant may be one with an anisotropyof dielectric constant Δε of 1 to 20, for example.Δε=(dielectric constant in the major axis direction)−(dielectricconstant in the minor axis direction)  (P)

In conventional liquid crystal display devices without any radicalscavengers, the defects, namely image sticking and stains, tend toappear more significantly in the case of a liquid crystal materialhaving negative anisotropy of dielectric constant than in the case of aliquid crystal material having positive anisotropy of dielectricconstant. This is presumably because large polarization appears in theminor axis direction in a liquid crystal material having negativeanisotropy of dielectric constant, with which the liquid crystal displaydevice becomes more susceptible to a decrease in VHR when the liquidcrystal molecules are ionized. That is, the radical scavenger used inthe present invention achieves a large effect in a system utilizing aliquid crystal material having negative anisotropy of dielectricconstant and photo-alignment films in combination.

The liquid crystal material preferably contains at least a compoundcontaining an alkenyl structure. Examples of the compound containing analkenyl structure include compounds represented by the following formula(D-1), (D-2), or (D-3).

In the formulas, m and n are the same or different integers, and areeach preferably 1 to 6.

Specific examples of the compounds containing an alkenyl structurerepresented by the above formula (D-1) include those represented by thefollowing formula (D-1-1).

The liquid crystal material preferably contains at least a compoundcontaining an alkoxy structure. Examples of the compound containing analkoxy structure include compounds represented by the following formula(E-1), (E-2), (E-3), (E-4), or (E-5).

In the formulas, m and n are the same or different integers, and areeach preferably 1 to 7.

Specific examples of the compounds containing an alkoxy structurerepresented by the above formula (E-3) include compounds represented bythe following formula (E-3-1).

<Radical Scavenger>

The radical scavenger may be any radical scavenger that is reactive withalignment film radicals generated from the photo-alignment films andwith liquid crystal radicals formed by transfer of the alignment filmradicals to the liquid crystal, and that thereby deactivates thealignment film radicals and the liquid crystal radicals. Suitableexamples thereof include hindered amine compounds. The hindered aminecompounds are regarded as having a tendency of being concentrated in thevicinities of the alignment films and being able to selectively reactwith alignment film radicals generated from the photo-alignment filmsbecause they are not much soluble in a liquid crystal material but arehighly compatible with amine and carboxylic acid present in thephoto-alignment films.

FIG. 2 is a view describing the reaction mechanism of deactivatingradicals generated from photo-alignment films with a hindered aminecompound (radical scavenger). As shown in the scheme (A-I) in FIG. 2, aphoto-functional group P_(AL) in the photo-alignment films is excited togenerate alignment film radicals R_(AL) when irradiated with light(amount of energy: hν). As shown in the scheme (A-II) in FIG. 2, ahindered amine compound added to the liquid crystal materialconstituting the liquid crystal layer 23 can selectively react with analignment film radical R_(AL) to deactivate the alignment film radicalR_(AL). Here, the hindered amine compound itself having reacted with analignment film radical R_(AL) is turned into a hindered amine radical.The R_(h) in the scheme (A-II) represents a hydrocarbon group derivedfrom the hindered amine compound. As shown in the scheme (A-III) in FIG.2, the hindered amine radical then binds to another alignment filmradical R_(AL), so that both the hindered amine radical and thealignment film radical R_(AL) disappear. Also, as shown in the scheme(A-IV) in FIG. 2, the compound formed as a result of binding of thehindered amine radical to the alignment film radical R_(AL) alsofunctions as a radical scavenger by further reacting with yet anotheralignment film radical R_(AL) to generate another hindered amineradical. The resulting hindered amine radical is also a radicalscavenger reactive with an alignment film radical R_(AL), as shown inthe scheme (A-III). Through these steps, a circulative cycle of scheme(A-I)→scheme (A-II)→scheme (A-III)→scheme (A-I)→scheme (A-IV)→scheme(A-III)→ and so forth proceeds in a system to which a hindered aminecompound (radical scavenger) is added. As a result, the radicalscavenger can keep deactivating the generated radicals without reductionin its amount, and therefore can keep inhibiting generation of ions fromthe radicals for a long period of time. That is, a small amount of thehindered amine compound (radical scavenger) enables reduction of adecrease in VHR due to exposure to the backlight illumination for a longperiod of time.

Also, the hindered amine compound, being highly reactive with radicalsgenerated from the photo-alignment films, can readily deactivate theradicals in the liquid crystal layer 23. Hence, in the case of using thehindered amine compound together with an antioxidant, the hindered aminecompound can effectively lessen the chance for the antioxidant to reactwith and be consumed by the radicals generated from the photo-alignmentfilms, and thus can also lessen the chance of generation of oxides inthe liquid crystal layer. Accordingly, the hindered amine compound canalso prevent image sticking and stains caused by oxides.

Furthermore, as a result of greatly reducing generation of ions in theliquid crystal layer 23 with a radical scavenger such as a hinderedamine compound, the frame duration in driving of the liquid crystaldisplay device can be lengthened. That is, a radical scavenger enablesdriving of the liquid crystal display device at a low frequency, andthereby enables reduction of the power consumption to a low level.

Examples of the hindered amine compound include compounds represented bythe following formula (F-1) or (F-2), with particularly suitableexamples being compounds represented by the following formula (F-1).

In the formulas, X represents a monovalent organic group and Rrepresents a hydrocarbon group.

Specific examples of the hindered amine compounds represented by theabove formula (F-1) include compounds represented by the followingformula (F-1-1), (F-1-2), (F-1-3), (F-1-4), or (F-1-5).

Specific examples of the hindered amine compounds represented by theabove formula (F-2) include compounds represented by the followingformula (F-2-1) or (F-2-2). The hindered amine compound may have astructure similar to the compounds represented by the above formula(F-2), such as a compound represented by the following formula (F-2-3)obtained by replacing the hydrogen atom with a methyl group.

The hindered amine compound may have a structure containing a radicalsite as shown in a compound represented by the following formula (F-3).Specific examples thereof include structures represented by thefollowing formula (F-3-1), (F-3-2), or (F-3-3).

The concentration of the radical scavenger (hindered amine compound) ispreferably in the range of 1 ppm to 1000 ppm. The radical scavengercontained at a concentration in such a range can sufficiently deactivateradicals generated from the photo-alignment films, particularlysufficiently achieving the effect of reducing a decrease in VHR. Here,since radicals of a hindered amine are stable, too high a concentrationof the hindered amine compound may not be suited for reduction of adecrease in VHR. For this reason, the later-described antioxidant mayalso be added instead of increasing the concentration of the hinderedamine compound. The upper limit of the concentration of the radicalscavenger (hindered amine compound) is more preferably 500 ppm, stillmore preferably 250 ppm.

<Antioxidant>

The liquid crystal layer 23 may further contain an antioxidant. Theantioxidant may be any antioxidant that is more reactive with oxygen oroxides than with the liquid crystal material. Suitable examples thereofinclude phenolic antioxidants.

FIG. 3 is a view describing the effect of a phenolic antioxidant in thepresent invention. As shown in the scheme (1) in FIG. 3, when oxygenenters the liquid crystal panel and the panel is exposed to light orheat energy, a structure such as an alkyl group (R) contained in theliquid crystal material, alignment films, and sealant is oxidized intoan oxidant (ROOH). The oxidant generates radicals which are ionized inthe absence of an antioxidant or radical scavenger. When the liquidcrystal material is oxidized and then ionized, ions are generated in theliquid crystal layer 23. In addition, also when the alignment films andsealant are oxidized, the oxidants dissociated from the polymersconstituting the alignment films and the sealant are ionized andreleased into the liquid crystal layer 23, so that ions are generated inthe liquid crystal layer 23. The ions in the liquid crystal layer 23decrease the VHR. In contrast, as shown in the schemes (2) and (3) inFIG. 3, an antioxidant added reacts with radicals before the radicalsare ionized, preventing generation of ions caused by oxidation of theliquid crystal material, photo-alignment films, and sealant. Also, sincethe amount of the antioxidant does not decrease in the cycles shown inthe schemes (2) and (3) in FIG. 3, the antioxidant can preventionization of radicals for a long period of time.

As shown in FIG. 3, the antioxidant undergoes the repeated cycle ofhydrogen group elimination→addition→elimination to eliminate (reduce)oxygen from oxides, and thereby reduce degradation (decomposition andionization) due to oxidation for a long period of time. Still, theantioxidant may be consumed in a reaction between the antioxidant andthe photo-alignment films, for example. FIG. 4 is a view describing thereaction between a phenolic antioxidant and a photo-reactive film. Asshown in FIG. 4, in the case that a cinnamate group (photo-functionalgroup) cleaves to generate radicals when irradiated with ultravioletlight from the backlight, the antioxidant reacts with any of theradicals and turns into an antioxidant radical. Here, the antioxidantradical may bind to the photo-alignment film side radical generated uponcleavage of the cinnamate group. In this case, since the antioxidanthaving bound to the photo-alignment film side radical cannot beconverted back to the antioxidant, the amount of the antioxidant in theliquid crystal layer 23 gradually decreases. Such consumption of theantioxidant for a long period of time may inhibit sufficient preventionof oxidation of the liquid crystal layer 23 and the photo-alignmentfilms. Although FIG. 4 illustrates the case of a cinnamate group, theconsumption of the antioxidant has been found to occur in the samemanner also in the case of another photo-functional group such as anazobenzene group. In order to prevent such consumption of theantioxidant, the present embodiment utilizes an antioxidant incombination with a radical scavenger. A radical scavenger traps radicalsin the alignment films and the liquid crystal regardless of whether theradicals are generated by oxides or not, and prevents ionization of theradicals by repeating trapping and releasing the radicals. With aradical scavenger that is more reactive with the radicals than theantioxidant and keeps trapping the radicals in the photo-alignment filmsand the liquid crystal, the antioxidant is presumably prevented frombeing consumed in the above reaction and can maintain the anti-oxidationfunction.

Examples of the phenolic antioxidants include those represented by thefollowing formula (G). Specific examples thereof include thoserepresented by the following formula (G-1), (G-2), or (G-3).

In the formula, X represents a monovalent organic group.

In the formulas, n represents an integer and is preferably an integer of3 to 20.

Specific examples of the phenolic antioxidants represented by the aboveformula (G) include compounds represented by the following formula(G-a), (G-b), (G-c), (G-d), (G-e), (G-f), or (G-g).

The concentration of the antioxidant is preferably in the range of 1 ppmto 10 wt %. The antioxidant contained at a concentration in such a rangecan prevent oxygen having entered the liquid crystal panel from theoutside from oxidizing the liquid crystal material, effectivelypreventing image sticking and stains due to oxides. Also, theantioxidant, as well as the radical scavenger, can partially deactivateradicals generated from the photo-alignment films, particularlysufficiently achieving the effect of reducing a decrease in VHR. Thelower limit of the concentration is more preferably 10 ppm, while theupper limit is more preferably 5 wt %, still more preferably 1 wt %.

The alignment mode of the liquid crystal panel is not particularlylimited, and may be, for example, a horizontal alignment mode such as afringe field switching (FFS) mode or an in-plane switching (IPS) mode; avertical alignment mode; or a twisted nematic (TN) mode.

In the case that the alignment mode of the liquid crystal panel is ahorizontal alignment mode, radicals are likely to be generated from thephoto-alignment films. Hence, the effect of adding a radical scavengercan be significant. More specifically, the photo-alignment treatment(irradiation with polarized UV light) in a vertical alignment mode isjust setting the pre-tilt angle direction to a direction slightly tiltedfrom 90°, while the photo-alignment treatment in a horizontal alignmentmode requires control of the azimuth direction (direction in thesubstrate plane) of the liquid crystal alignment with a higherprecision. The irradiation dose in the photo-alignment treatment in thehorizontal alignment mode is therefore usually one or more digitsgreater than that in the vertical alignment mode, and the treatment isaccompanied by an adverse effect that a larger number of radicals arelikely to be generated than that in the vertical alignment mode. Sincethe radical scavenger contained in the liquid crystal layer candeactivate the radicals generated in the photo-alignment treatment, theradical scavenger can effectively prevent the radicals from remaining inthe completed liquid crystal panel (after injection of liquid crystal).

In the FFS mode, at least one of the substrates 21 includes a structure(FFS electrode structure) with a planar electrode, slit electrodes, andan insulating film disposed between the planar electrode and the slitelectrodes, so that oblique electric fields (fringe electric fields) aregenerated in the liquid crystal layer 23 adjacent to the substrate 21.Typically, the components are disposed in the order from the liquidcrystal layer 23 side of the slit electrodes, the insulating film, andthe planar electrode. Each slit electrode may be, for example, a slitelectrode including linear openings (slits) entirely surrounded by theelectrode, or a comb-shaped slit electrode provided with comb teeth andlinearly cut portions (slits) that are formed between the comb teeth.

In the IPS mode, at least one of the substrates 21 includes paired combelectrodes which generate transverse electric fields in the liquidcrystal layer 23 adjacent to the substrate 21. The paired combelectrodes may be, for example, electrodes that are each including combteeth and are disposed with the comb teeth of the respective electrodesbeing engaged with each other.

In the liquid crystal panel 20 of the present embodiment, typically, thepaired substrates 21 are attached to each other with the sealant (notillustrated) disposed to surround the liquid crystal layer 23, so thatthe liquid crystal layer 23 is held in the given region. The sealant maybe, for example, an epoxy resin containing inorganic or organic fillerand a curing agent.

The paired substrates 21 each may be provided with a polarizer (linearpolarizer) on the side opposite to the liquid crystal layer 23. Typicalexamples of the polarizer include those obtained by aligning a dichroicanisotropic material such as an iodine complex adsorbed on a polyvinylalcohol (PVA) film. Generally, each surface of the PVA film is laminatedwith a protective film such as a triacetyl cellulose film before thefilm is put into practical use. An optical film such as a retardationfilm may be disposed between the polarizer and each of the substrates21.

As illustrated in FIG. 1, the liquid crystal display device of thepresent embodiment is provided with the backlight 10 on the back surfaceside of the liquid crystal panel. A liquid crystal display device withsuch a configuration is usually called a transmissive liquid crystaldisplay device. The backlight 10 may be any backlight that emits lightincluding visible light, and may be one that emits light with onlyvisible light or emits light including both visible light andultraviolet light. In order to enable the liquid crystal display deviceto provide color display, a backlight emitting white light is suitablefor the backlight 10. The suitable types of the backlight 10 include,for example, light emitting diodes (LEDs). The “visible light” as usedherein refers to light (electromagnetic waves) having a wavelength of380 nm to shorter than 800 nm.

The present invention has a feature in deactivating, with a radicalscavenger, radicals generated from the photo-alignment films underexposure to the backlight 10 illumination. The radical scavenger cantherefore be effectively used in the case that the emission spectrum ofthe backlight 10 at least partially overlaps the absorption spectra ofthe photo-alignment films.

The liquid crystal display device of the present embodiment has aconfiguration including, as well as the liquid crystal panel 20 and thebacklight 10, components such as external circuits such as atape-carrier package (TCP) and a printed circuit board (PCB); opticalfilms such as a viewing angle-increasing film and a luminance-increasingfilm; and a bezel (frame). Some components, if appropriate, may beincorporated into another component. In addition to the componentsdescribed above, the liquid crystal display device may include anycomponents that are usually used in the field of liquid crystal displaydevices. The additional components are therefore not described here.

Each and every detail described for the above embodiment of the presentinvention shall be applied to all the aspects of the present invention.

The present invention is described below in more detail based onexamples and comparative examples. The examples, however, are notintended to limit the scope of the present invention.

Example 1

A liquid crystal panel in the fringe field switching mode (FFS mode) wasactually produced by the following method.

First, a TFT substrate including components such as TFTs and FFSelectrode structures and a color filter substrate (CF substrate)including components such as a black matrix and color filters wereprepared. To the surface of each of the TFT substrate and the CFsubstrate was applied an alignment film solution. The solids content ofthe alignment film solution was a polymer material containing a polyamicacid structure and a photo-reactive azobenzene structure in the mainchain.

In order to volatilize the solvent in the alignment film solution, thesubstrates were heated at 70° C. Subsequently, the photo-alignmenttreatment was performed by irradiating the surfaces of the substrateswith linearly polarized light having a dominant wavelength of 365 nmwith an intensity of 2000 mJ/cm². The polarization direction of thelinearly polarized light was set to be orthogonal to the alignmentdirection of the liquid crystal. The azobenzene structure, whenirradiated with linearly polarized light, underwent a trans-cisisomerization reaction, thereby exerting the alignment force. Thetrans-azobenzene has a structure represented by the following formula(B-5-1), and the cis-azobenzene has a structure represented by thefollowing formula (B-5-2).

The substrates were then heated (post-baked) at 220° C. The post-bakingcaused partial imidization (cyclodehydration) of the polyamic acidstructure to generate a polyimide structure. Thereby, horizontalalignment films exerting a sufficient alignment force were obtained byphoto-irradiation. The post-baked alignment films each had a thicknessof 100 nm.

A liquid crystal composition was dropped on the TFT substrate, while aheat/visible light-curable sealant was poured with a dispenser on the CFsubstrate. The TFT substrate and the CF substrate were then attached toeach other, so that the liquid crystal composition was sealed betweenthe substrates. During attachment of the substrates, the sealant wasexposed to light for curing, with the display region shielded fromlight.

The liquid crystal composition used was one obtained by adding ahindered amine compound (radical scavenger) represented by the followingformula (F-1-2) to a liquid crystal material containing a compound withan alkenyl structure represented by the following formula (D-1-1). Theconcentration of the hindered amine compound in the whole liquid crystalcomposition was 200 ppm. The liquid crystal material had negativeanisotropy of dielectric constant (Δε=−3.5).

The liquid crystal composition was then heated at 130° C. for 40minutes, and thereby the liquid crystal molecules were re-aligned.Paired polarizers were attached to the back surface side of the TFTsubstrate (backlight illumination incident surface side) and the viewingside of the CF substrate (backlight illumination emission surface side),with the polarization axes being arranged in crossed Nicols. Thereby, aFFS mode liquid crystal panel was produced. A backlight with white LEDswas then mounted on the back surface side of the liquid crystal panel,whereby a liquid crystal display device of Example 1 was completed.

Comparative Example 1

A FFS mode liquid crystal panel was produced by the same procedure asthat in Example 1 except that the hindered amine compound was not addedto the liquid crystal composition.

Comparative Example 2

A FFS mode liquid crystal panel was produced by the same procedure asthat in Example 1, except that the photo-alignment films exerting analignment force by photo-irradiation were replaced by rubbed alignmentfilms exerting an alignment force by rubbing treatment, and that thehindered amine compound was not added to the liquid crystal composition.

In Comparative Example 2, the solids content in the alignment filmsolution was a polymer material containing a polyamic acid structure inthe main chain. Also, the photo-alignment treatment was not performed,and the rubbing treatment was performed.

(Evaluation Test 1)

The liquid crystal panels produced in Example 1 and Comparative Examples1 and 2 each were kept being electrically charged with the backlight on,such that changes over time in the voltage holding ratio weredetermined. FIG. 5 is a graph showing changes over time in the voltageholding ratio of the liquid crystal panels of Example 1 and ComparativeExamples 1 and 2.

As shown in FIG. 5, comparison between Example 1 and Comparative Example1 shows that addition of the hindered amine compound to the liquidcrystal material reduced the decrease in the voltage holding ratio.Comparison between Comparative Example 1 and Comparative Example 2 showsthat the significant decrease in the voltage holding ratio inComparative Example 1 was due to the photo-alignment films. In otherwords, the results clearly show that the effects obtained in Example 1can be achieved more markedly with a combination of the radicalscavenger (hindered amine compound) and the photo-alignment films.

The decrease in the voltage holding ratio due to the photo-alignmentfilms was presumably caused by the following mechanism.

The azobenzene structure contained in the photo-alignment films used inExample 1 and Comparative Example 1 is alignment-treated by light havinga wavelength of 365 nm which is in a region near the visible lightregion. Meanwhile, the backlight of the liquid crystal display devicemainly emits light including visible light for color display. Here, theresults of Comparative Example 1 suggest that the short wavelength sideof the backlight emission spectrum and the long wavelength side of theabsorption spectrum of the azobenzene structure slightly overlapped eachother, though the overlap may not have been easily detected in an actualspectrum analysis, and thus radicals were generated. For example, asshown in the following reaction scheme, the backlight illumination mayhave caused a light cleavage reaction of the azobenzene structure. Incontrast, the results of Example 1 show that the hindered amine compoundeffectively deactivated the radicals generated in the reaction of thephoto-alignment films, preventing a decrease in the voltage holdingratio.

Photo-reactive sites that are alignment-treated by light having awavelength in a region near the visible light region similarly to theazobenzene structure include, for example, structures such as cinnamate,chalcone, coumarin, stilbene, and phenolic esters. All of thesephoto-reactive sites are considered to absorb, though slightly, lighthaving a wavelength of 340 nm or longer. Hence, they can also absorb thebacklight illumination to generate radicals, similarly to the azobenzenestructure. For example, cinnamate, chalcone, and phenolic esters undergophoto-Fries rearrangement (ester group cleavage) to generate radicals,while chalcone undergoes hydrogen abstraction or light cleavage togenerate radicals as shown in the following reaction scheme.Accordingly, also in the case of using photo-alignment films with any ofthese photo-reactive sites, a radical scavenger such as a hindered aminecompound is preferably added to the liquid crystal material.

Also, a liquid crystal component containing an alkenyl structure iseffective in decreasing the viscosity of the liquid crystal material.However, the double bond in an alkenyl structure is vulnerable toradical attacks, and therefore tends to be a cause of a decrease in VHRwhen such a liquid crystal component is used in combination with thephoto-alignment films that can be a source of radicals. In Example 1,the addition of a hindered amine compound to the liquid crystal materialenabled effective prevention of radical attacks to the alkenylstructure. For an increase in the response rate of the liquid crystaldisplay device, the liquid crystal component containing an alkenylstructure is preferably added not only to a liquid crystal materialhaving negative anisotropy of dielectric constant but also to a liquidcrystal material having positive anisotropy of dielectric constant.

Example 2

A liquid crystal display device including a FFS mode liquid crystalpanel was actually produced by the following method.

First, a TFT substrate including components such as TFTs and FFSelectrode structures and a CF substrate including components such as ablack matrix and color filters were prepared. To the surface of each ofthe TFT substrate and the CF substrate was applied an alignment filmsolution. The solids content of the alignment film solution was apolymer material containing a polyamic acid structure obtained bypolymerizing an acid anhydride represented by the following formula(B-7-1) and an amine compound represented by the following formula(B-7-2). Here, hydrogen atoms in cyclobutane in the acid anhydriderepresented by the following formula (B-7-1) may each be replaced byanother atom or functional group.

In order to volatilize the solvent in the alignment film solution, thesubstrates were heated at 70° C. Subsequently, the substrates wereheated (post-baked) at 230° C. The post-baking caused partialimidization (cyclodehydration) of the polyamic acid structure togenerate a polyimide structure. The photo-alignment treatment was thenperformed by irradiating the surfaces of the substrates with linearlypolarized light having a dominant wavelength of 254 nm with an intensityof 600 mJ/cm². The polarization direction of the linearly polarizedlight was set to be orthogonal to the alignment direction of the liquidcrystal. As shown in the following scheme (B-7-I), the cyclobutane site,when irradiated with linearly polarized light, underwent a decompositionreaction and cleaved. The cleavage caused dissipation of the alignmentforce in the polymer chain direction, causing exertion of the alignmentforce in an azimuth direction orthogonal to the dissipated force.Thereby, horizontal alignment films exerting a sufficient alignmentforce were obtained by photo-irradiation. The post-baked alignment filmseach had a thickness of 100 nm.

A liquid crystal composition was dropped on the TFT substrate, while aheat/visible light-curable sealant was poured with a dispenser on the CFsubstrate. The TFT substrate and the CF substrate were then attached toeach other, so that the liquid crystal composition was sealed betweenthe substrates. During attachment of the substrates, the sealant wasexposed to light for curing, with the display region shielded fromlight.

The liquid crystal composition used was one obtained by adding the samehindered amine compound (radical scavenger) represented by the aboveformula (F-1-2) as that in Example 1 to a liquid crystal materialcontaining a compound with an alkoxy structure represented by thefollowing formula (E-3-1). The concentration of the hindered aminecompound in the whole liquid crystal composition was 200 ppm. The liquidcrystal material had negative anisotropy of dielectric constant(Δε=−3.5).

The liquid crystal composition was then heated at 130° C. for 40minutes, and thereby the liquid crystal molecules were re-aligned.Paired polarizers were attached to the back surface side of the TFTsubstrate (backlight illumination incident surface side) and the viewingside of the CF substrate (backlight illumination emission surface side),with the polarization axes being arranged in crossed Nicols. Thereby, aFFS mode liquid crystal panel was produced. A backlight with white LEDswas then mounted on the back surface side of the liquid crystal panel,whereby a liquid crystal display device of Example 2 was completed.

Comparative Example 3

A liquid crystal display device of Comparative Example 3 was produced bythe same procedure as that in Example 2, except that the hindered aminecompound was not added to the liquid crystal composition.

(Evaluation Test 2)

The liquid crystal display devices produced in Example 2 and ComparativeExample 3 each were kept being electrically charged with the backlighton. Here, a white box pattern was displayed to the liquid crystaldisplay device screen with a black background. After 500 hours, thedisplayed image was entirely changed to a gray image at a grayscalevalue of 64. As a result, the liquid crystal display device ofComparative Example 3 caused stain-like image sticking around the boxpattern. In contrast, the liquid crystal display device of Example 2 didnot cause such image sticking.

As shown in the following reaction scheme, the cyclobutane structurecontained in the photo-alignment films used in Example 2 usually mainlyabsorbs light having a wavelength of 300 nm or shorter and generatesradicals in an intermediate stage of the reaction. Yet, suchphoto-alignment films containing a cyclobutane structure may be modifiedto contain a structure with a high light absorbance for reduction of theexposure dose in the alignment treatment. For example, a skeleton with ahigh light absorbance may be selected for a diamine site and theabsorbed light energy may be transferred to the cyclobutane site topromote the light cleavage of the cyclobutane site. Such modificationmay increase the light absorbance of the photo-alignment films for lighthaving a longer wavelength, but may cause the short wavelength side ofthe emission spectrum of the backlight to overlap the long wavelengthside of the absorption spectra of the photo-alignment films. Also, sincethe exposure dose in the alignment treatment is as high as severalhundreds of millijoules or higher per square centimeter, some radicalsgenerated in the alignment treatment may not yet be deactivated afterthe completion of the liquid crystal panel. For this reason,decomposition-type photo-alignment films containing a cyclobutanestructure also include a cause of image sticking, and thus the liquidcrystal display device of Comparative Example 3 caused image sticking.The results of Example 2 show that the hindered amine compoundeffectively deactivated radicals generated in the reaction of thephoto-alignment films, and thereby prevented image sticking.

The alkoxy structure in the liquid crystal material used in Example 2 issuitable for controlling the anisotropy of dielectric constant of aliquid crystal material (negative liquid crystal) having negativeanisotropy of dielectric constant. As to liquid crystal materials(positive liquid crystal) having positive anisotropy of dielectricconstant, the anisotropy of dielectric constant can be easily controlledwithout use of alkoxy groups. In a conventional liquid crystal displaydevice, use of a liquid crystal material containing an alkoxy structuretends to decrease the VHR, and the tendency was significant especiallywhen such a liquid crystal material is combined with photo-alignmentfilms. Such a decrease in VHR can be reduced by adding a radicalscavenger as in the present invention. The reason therefor can beexplained based on the following Hypothetical Models 1 to 4.

[Hypothetical Model 1]

As shown in the following reaction scheme, an alkoxy structure (—OR) isvulnerable to the attack of a radical R_(AL) generated from aphoto-alignment film, and undergoes any of the four radical generationreaction patterns. When the generated radicals are ionized, the VHR isdecreased.

[Hypothetical Model 2]

As shown in the following reaction scheme, the radical R_(AL) generatedfrom a photo-alignment film binds to oxygen in the liquid crystal layerto form a peroxide structure (ROO.). The alkoxy structure (—OR) isvulnerable to the attack of the peroxide structure, and undergoes any ofthe five radical generation reaction patterns. Also in each pattern, theradical generation reactions occur in chains, with one radicalgeneration reaction followed by another radical generation reaction.When the generated radicals are ionized, the VHR is decreased. Thisradical chain reaction with the peroxide structure is known as anautoxidation reaction.

[Hypothetical Model 3]

Alkoxy structures (especially methoxy and ethoxy groups) areelectron-donating groups and are converted to their resonance structuresunder light exposure. The following scheme shows a part of a compoundwith an alkoxy structure, with three resonance structures of the alkoxystructure. Among these, the resonance structure (a) shown at the centerand the resonance structure (b) shown on the right are ionized and canbe the causes of a decrease in VHR. Furthermore, the resonancestructures (a) and (b) are respectively converted into structures (a′)and (b′) each having a peroxide structure in the presence of oxygen. Thestructures (a′) and (b′) each having a peroxide structure respectivelyeasily generate radicals represented by the structures (a″) and (b″).When the generated radicals are ionized, the VHR is decreased.

[Hypothetical Model 4]

Negative liquid crystal containing an alkoxy structure consists ofmolecular structures with large polarization. The negative liquidcrystal therefore has a higher solubility of impurity ions and is morelikely to contain mobile ions than positive liquid crystal. Since mobileions have an effect of canceling out the charges, the VHR is decreased.

Since the above Hypothetical Models 1 to 3 involve radicals, the modelscan be dealt by employing a radical scavenger to trap the radicals.Hypothetical Model 4 describes that the ionic impurities generatedthrough radical generation have a greater influence on the negativeliquid crystal than on the positive liquid crystal, which suggests thattrapping radicals indirectly solves the problem of Hypothetical Model 4.Consequently, addition of a radical scavenger to a liquid crystal layercan achieve an effect of reducing a decrease in VHR caused by use of aliquid crystal material containing an alkoxy structure.

Example 3

A liquid crystal display device including a FFS mode liquid crystalpanel was actually produced by the following method.

First, a TFT substrate including components such as TFTs and FFSelectrode structures and a CF substrate including components such as ablack matrix and color filters were prepared. To the surface of each ofthe TFT substrate and the CF substrate was applied an alignment filmsolution. The solids content of the alignment film solution was apolymer material containing a polysiloxane structure as the mainskeleton and a cinnamate group represented by the following formula(B-1) functioning as a photo-functional group in a side chain.

In order to volatilize the solvent in the alignment film solution, thesubstrates were heated at 70° C. Subsequently, the substrates wereheated (post-baked) at 230° C. The photo-alignment treatment was thenperformed by irradiating the surfaces of the substrates with linearlypolarized light having a dominant wavelength of 313 nm with an intensityof 200 mJ/cm². The polarization direction of the linearly polarizedlight was set to be orthogonal to the alignment direction of the liquidcrystal. The cinnamate group, when irradiated with linearly polarizedlight, underwent isomerization and dimerization reactions, exerting thealignment force. Thereby, horizontal alignment films exerting asufficient alignment force were obtained by photo-irradiation. Thepost-baked alignment films each had a thickness of 100 nm.

A liquid crystal composition was dropped on the TFT substrate, while aheat/visible light-curable sealant was poured with a dispenser on the CFsubstrate. The TFT substrate and the CF substrate were then attached toeach other, so that the liquid crystal composition was sealed betweenthe substrates. During attachment of the substrates, the sealant wasexposed to light for curing, with the display region shielded fromlight.

The liquid crystal composition used was one obtained by adding ahindered amine compound (radical scavenger) represented by the followingformula (F-1-5) and an antioxidant represented by the following formula(G-g) to a liquid crystal material containing a compound with the samealkenyl structure represented by the above formula (D-1-1) as that inExample 1. The concentration of the hindered amine compound in the wholeliquid crystal composition was 200 ppm. The concentration of theantioxidant in the whole liquid crystal composition was 0.1 wt %. Theliquid crystal material had positive anisotropy of dielectric constant(Δε=+9.0).

The liquid crystal composition was then heated at 130° C. for 40minutes, and thereby the liquid crystal molecules were re-aligned.Paired polarizers were attached to the back surface side of the TFTsubstrate (backlight illumination incident surface side) and the viewingside of the CF substrate (backlight illumination emission surface side),with the polarization axes being arranged in crossed Nicols. Thereby, aFFS mode liquid crystal panel was produced. A backlight with white LEDswas then mounted on the back surface side of the liquid crystal panel,whereby a liquid crystal display device of Example 3 was completed.

Comparative Example 4

A liquid crystal display device of Comparative Example 4 was produced bythe same procedure as that in Example 3, except that the hindered aminecompound and the antioxidant were not added to the liquid crystalcomposition.

(Evaluation Test 3)

The liquid crystal display devices produced in Example 3 and ComparativeExample 4 each were kept being electrically charged with the backlighton. Here, a white image was displayed to the entire screen of the liquidcrystal display device. After 500 hours, the displayed image wasentirely changed to a gray image at a grayscale value of 64. As aresult, the liquid crystal display device of Comparative Example 4caused stain-like unevenness at the edges of the screen. This unevennessis presumably caused by a decrease in VHR. In contrast, the liquidcrystal display device of Example 3 did not cause such unevenness.

Example 4

A liquid crystal display device including a FFS mode liquid crystalpanel was actually produced by the following method.

First, a TFT substrate including components such as TFTs and FFSelectrode structures and a CF substrate including components such as ablack matrix and color filters were prepared. To the surface of each ofthe TFT substrate and the CF substrate was applied an alignment filmsolution. The solids content of the alignment film solution was apolymer material containing a polysiloxane structure as the mainskeleton and a cinnamate group functioning as a photo-functional groupin a side chain.

In order to volatilize the solvent in the alignment film solution, thesubstrates were heated at 70° C. Subsequently, the substrates wereheated (post-baked) at 230° C. The photo-alignment treatment was thenperformed by irradiating the surfaces of the substrates with linearlypolarized light having a dominant wavelength of 313 nm with an intensityof 20 mJ/cm². The polarization direction of the linearly polarized lightwas set to be orthogonal to the alignment direction of the liquidcrystal. The cinnamate group, when irradiated with linearly polarizedlight, underwent isomerization and dimerization reactions, exerting thealignment force. Thereby, horizontal alignment films exerting analignment force were obtained by photo-irradiation. The post-bakedalignment films each had a thickness of 100 nm. In the present example,the exposure dose in the photo-alignment treatment was set lower thanthat in Example 3 utilizing the same alignment film solution. Still, thealignment force was enhanced by polymerizing a photo-polymerizablemonomer added to the liquid crystal material on the surfaces of thealignment films as described below.

A liquid crystal composition was dropped on the TFT substrate, while aheat/visible light-curable sealant was poured with a dispenser on the CFsubstrate. The TFT substrate and the CF substrate were then attached toeach other, so that the liquid crystal composition was sealed betweenthe substrates. During attachment of the substrates, the sealant wasexposed to light for curing, with the display region shielded fromlight.

The liquid crystal composition used was one obtained by adding aphoto-polymerizable monomer represented by the following formula(C-1-1), the same hindered amine compound (radical scavenger)represented by the above formula (F-1-5) as that in Example 3, and theantioxidant represented by the above formula (G-g) to a liquid crystalmaterial containing a compound with the same alkenyl structurerepresented by the above formula (D-1-1) as that in Example 1. Theproportion of the photo-polymerizable monomer in the whole liquidcrystal composition was 0.25 wt %. The concentration of the hinderedamine compound in the whole liquid crystal composition was 200 ppm. Theconcentration of the antioxidant in the whole liquid crystal compositionwas 0.1 wt %. The liquid crystal material had negative anisotropy ofdielectric constant (Δε=−3.5).

Here, the photo-polymerizable monomer may be a monomer other than themonomer represented by the above formula (C-1-1). For example, a monomerrepresented by the above formula (C-1-2) obtained by substituting anacrylate group for the terminal methacrylate group in the monomerrepresented by the formula (C-1-1), or a monomer represented by theabove formula (C-1-3) obtained by substituting phenanthrene for theskeleton in the monomer represented by the formula (C-1-1) may be used.Also, in the formulas (C-1-1), (C-1-2), and (C-1-3), hydrogen atomspresent in the skeleton may each independently be replaced by a halogenatom.

After the sealant was cured, the display region of the liquid crystalpanel was irradiated with black light with an intensity of 3000 mJ/cm².Thereby, the photo-polymerizable monomer in the liquid crystal layer waspolymerized on the surfaces of the alignment films by taking in theliquid crystal molecules. As a result, the liquid crystal alignment onthe surfaces of the alignment films was fixed by the polymer of thephoto-polymerizable monomer, whereby a sufficient alignment force wasachieved.

The liquid crystal composition was then heated at 130° C. for 40minutes, and thereby the liquid crystal molecules were re-aligned.Paired polarizers were attached to the back surface side of the TFTsubstrate (backlight illumination incident surface side) and the viewingside of the CF substrate (backlight illumination emission surface side),with the polarization axes being arranged in crossed Nicols. Thereby, aFFS mode liquid crystal panel was produced. A backlight with white LEDswas then mounted on the back surface side of the liquid crystal panel,whereby a liquid crystal display device of Example 4 was completed.

Comparative Example 5

A liquid crystal display device of Comparative Example 5 was produced bythe same procedure as that in Example 4, except that the hindered aminecompound and the antioxidant were not added to the liquid crystalcomposition.

(Evaluation Test 4)

The liquid crystal display devices produced in Example 4 and ComparativeExample 5 each were kept being electrically charged with the backlighton. Here, a white image was displayed to the entire screen of the liquidcrystal display device. After 500 hours, the displayed image wasentirely changed to a gray image at a grayscale value of 64. As aresult, the liquid crystal display device of Comparative Example 5caused stain-like unevenness at the edges of the screen. This unevennessis presumably caused by a decrease in the voltage holding ratio. Incontrast, the liquid crystal display device of Example 4 did not causesuch unevenness.

The photo-polymerizable monomer used in Example 4 and ComparativeExample 5 can be a source of radicals. Hence, in Example 4 andComparative Example 5 with the sources of radicals, namely thephoto-polymerizable monomer and the photo-alignment films, radicals arelikely to be generated in the liquid crystal layer. Such radicalsgenerated in the reaction of the photo-alignment films andphoto-polymerizable monomers remaining after the PSA treatment can beeffectively deactivated by adding a hindered amine compound to theliquid crystal material. Also, the same effect can be achieved by addingan antioxidant to the liquid crystal material. As described above, theliquid crystal display device of Comparative Example 5 causedunevenness, but the liquid crystal display device of Example 4effectively prevented unevenness.

ADDITIONAL REMARKS

One aspect of the present invention may be a liquid crystal displaydevice, including: an active-matrix liquid crystal panel; and abacklight. The liquid crystal panel may include a liquid crystal layer,paired substrates holding the liquid crystal layer in between, andalignment films disposed on the liquid crystal layer side surfaces ofthe respective paired substrates. The alignment films may each be aphoto-alignment film formed from a material exhibiting photo-alignmentperformance. The liquid crystal layer may contain a liquid crystalmaterial and a radical scavenger. This aspect enables deactivation ofradicals released to the liquid crystal layer with a radical scavenger,preventing a decrease in VHR. Thereby, the liquid crystal displaydevice, with the photo-alignment films, can maintain a favorable VHR fora long period of time and prevent generation of image sticking andstains on the display screen.

The radical scavenger preferably contains a compound represented by thefollowing formula (1). A hindered amine compound represented by thefollowing formula (1) used as the radical scavenger can keepdeactivating radicals based on a circulative cycle. Therefore, a smallamount of the hindered amine compound enables reduction of a decrease inVHR due to exposure to the backlight illumination for a long period oftime. Also, the hindered amine compound, being highly reactive withradicals generated from the photo-alignment films, can readilydeactivate the radicals in the liquid crystal layer.

In the formula, X represents a monovalent organic group, and Rrepresents a hydrocarbon group.

The liquid crystal layer may further contain an antioxidant. Suitableexamples of the antioxidant include compounds represented by thefollowing formula (2). The antioxidant can prevent a decrease in VHR dueto radicals generated from oxidants that are generated through oxidationof a structure such as an alkyl group (R) contained in the liquidcrystal material, alignment films, and sealant by oxygen having enteredthe liquid crystal panel.

In the formula, X represents a monovalent organic group.

Examples of the photo-alignment film include those containing at leastone photo-reactive site selected from the group consisting of cinnamate,chalcone, coumarin, stilbene, azobenzene, and phenolic esters. Also, thephoto-alignment film may be formed from a polymer obtained bypolymerizing a monomer that contains an acid anhydride represented bythe following formula (3). The long wavelength side of the absorptionspectrum of each of these photo-alignment films overlaps the shortwavelength side of the backlight emission spectrum, and thus radicalsare generated upon irradiation with the backlight illumination. Hence,the effect of preventing a decrease in VHR can be effectively achievedwith a radical scavenger.

In the formula, hydrogen atoms may each be replaced.

The liquid crystal material may contain at least a compound containingan alkenyl structure. Examples of the compound containing an alkenylstructure include compounds represented by the following formula (4-1),(4-2), or (4-3). Although a liquid crystal component containing analkenyl structure is effective in decreasing the viscosity of the liquidcrystal material, the double bond in an alkenyl structure is vulnerableto radical attacks. Hence, the effect of preventing a decrease in VHRcan be effectively achieved with a radical scavenger.

In the formulas, m and n are the same or different integers.

The liquid crystal material may have negative anisotropy of dielectricconstant. In conventional liquid crystal display devices, the defects,namely image sticking and stains, tend to appear more significantly inthe case of a liquid crystal material having negative anisotropy ofdielectric constant than in the case of a liquid crystal material havingpositive anisotropy of dielectric constant. Hence, the effect ofpreventing a decrease in VHR can be effectively achieved with a radicalscavenger.

The liquid crystal material may contain at least a compound containingan alkoxy structure. Examples of the compound containing an alkoxystructure include compounds represented by the following formula (5-1),(5-2), (5-3), (5-4), or (5-5). Since the resonance structures of thealkoxy structures (especially methoxy and ethoxy groups) include ionizedones, they can be a cause of a decrease in VHR. Hence, the effect ofpreventing a decrease in VHR can be effectively achieved with a radicalscavenger.

In the formulas, m and n are the same or different integers.

Suitable alignment modes of the liquid crystal panel are a fringe fieldswitching mode and an in-plane switching mode. The photo-alignmenttreatment in a horizontal alignment mode requires control of the azimuthdirection of the liquid crystal alignment with a high precision. Theirradiation dose in the photo-alignment treatment in the horizontalalignment mode is therefore usually one or more digits greater than thatin the vertical alignment mode, and the treatment is accompanied by anadverse effect that a larger number of radicals are likely to begenerated than that in the vertical alignment mode. Hence, the effect ofpreventing a decrease in VHR can be effectively achieved with a radicalscavenger.

The liquid crystal panel may include, on the liquid crystal layer sidesurface of each alignment film, a layer containing a polymer that isobtained by polymerizing a photo-polymerizable monomer represented bythe following formula (6). Examples of Y in the following formula (6)include structures represented by the following formula (7-1), (7-2), or(7-3). In the case of adding a photo-polymerizable monomer to the liquidcrystal layer for the PSA treatment, the photo-polymerizable monomer aswell as the photo-alignment films is the source of radicals, andtherefore radicals are more likely to be generated in the liquid crystallayer. Hence, the effect of preventing a decrease in VHR can beeffectively achieved with a radical scavenger.A1-Y-A2  (6)In the formula, Y represents a structure containing at least one benzenering and/or a condensed benzene ring; hydrogen atoms in the benzene ringand the condensed benzene ring may each be replaced by a halogen atom;at least one of A1 and A2 represents an acrylate or a methacrylate; andA1 and A2 each directly bind to the benzene ring or the condensedbenzene ring.

In the formulas, hydrogen atoms may each be replaced by a halogen atom.

The aspects of the present invention described above may appropriatelybe combined within the spirit of the present invention.

REFERENCE SIGNS LIST

-   10: backlight-   20: liquid crystal panel-   21: substrate-   22: alignment film-   23: liquid crystal layer

The invention claimed is:
 1. A liquid crystal display device comprising:an active-matrix liquid crystal panel; and a backlight, the liquidcrystal panel including a liquid crystal layer, paired substratesholding the liquid crystal layer in between, and alignment filmsdisposed on the liquid crystal layer side surfaces of the respectivesubstrates, the alignment films each being a photo-alignment film formedfrom a material exhibiting photo-alignment performance, the liquidcrystal layer containing a liquid crystal material and a radicalscavenger; wherein the radical scavenger contains a compound representedby the following formula (1):

where X represents a monovalent organic group, and R represents ahydrocarbon group; and the photo-alignment films each are formed from apolymer obtained by polymerizing a monomer that contains a polyamic acidstructure and a photo-reactive azobenzene structure in the main chain.2. The liquid crystal display device according to claim 1, wherein theliquid crystal layer further contains an antioxidant.
 3. The liquidcrystal display device according to claim 2, wherein the antioxidantcontains a compound represented by the following formula (2):

wherein X represents a monovalent organic group.
 4. The liquid crystaldisplay device according to claim 1, wherein the liquid crystal materialcontains at least a compound containing an alkenyl structure.
 5. Theliquid crystal display device according to claim 4, wherein the compoundcontaining an alkenyl structure is represented by the following formula(4-1), (4-2), or (4-3):

wherein m and n are the same or different integers.
 6. The liquidcrystal display device according to claim 1, wherein the liquid crystalmaterial has negative anisotropy of dielectric constant.
 7. The liquidcrystal display device according to claim 6, wherein the liquid crystalmaterial contains at least a compound containing an alkoxy structure. 8.The liquid crystal display device according to claim 7, wherein thecompound containing an alkoxy structure is represented by the followingformula (5-1), (5-2), (5-3), (5-4), or (5-5):

wherein m and n are the same or different integers.
 9. The liquidcrystal display device according to claim 1, wherein the liquid crystalpanel includes, on the liquid crystal layer side surface of eachalignment film, a layer containing a polymer that is obtained bypolymerizing a photo-polymerizable monomer represented by the followingformula (6):A1-Y-A2  (6) wherein Y represents a structure containing at least onebenzene ring and/or a condensed benzene ring; hydrogen atoms in thebenzene ring and the condensed benzene ring may each be replaced by ahalogen atom; at least one of A1 and A2 represents an acrylate or amethacrylate; and A1 and A2 each directly bind to the benzene ring orthe condensed benzene ring.
 10. The liquid crystal display deviceaccording to claim 9, wherein Y in the above formula (6) is a structurerepresented by the following formula (7-1), (7-2), or (7-3):

wherein hydrogen atoms may each be replaced by a halogen atom.